Viscoelastic materials, such as elastomers, rubbers, plastics, and the like, can be described by a number of fundamental viscoelastic properties which vary between different material types and, in fact, may vary between separate batches of the same material type. Consequently, these fundamental properties, such as the loss and storage moduli, the complex viscosity and the loss tangent, may be used to identify materials either by type or by grade. Further, these properties may also be used to predict the processing characteristics of a material.
However, heretofore many methods of determining the fundamental viscoelastic properties of materials have been time consuming, i.e., often requiring several hours, and have relied on the participation of a skilled technician to perform the test. Moreover, with many tests it is possible only to describe a few characteristics of the material, such as die swell or stress relaxation, and not actually to determine the fundamental viscoelastic properties of the material. These drawbacks have made these tests unsuitable for use on the factory floor or for use in a control system wherein the results of the test are employed to modify the composition or processing of the material.
A Dynamic Stress Relaxometer (DSR), as described in U.S. Pat. Nos. 3,693,421 and 3,818,751, has provided a relatively quick way to predict some characteristics of a material and also to at least roughly distinguish between materials and grades of materials. The disclosures of such patents hereby are entirely incorporated by reference. In a DSR test a sample of a viscoelastic material is subjected to a prescribed angular deformation thus creating shear stresses in the material. The gradual relaxation of these stresses over time is measured and plotted to provide a stress relaxation curve for the material. As the relaxation curve follows a generally exponential decay path, a number of standard measurement points along the curve are selected which have been found to yield significant information about the test specimen in a relatively short time, i.e., less than ten minutes. This information is then used to predict the die swell, processability or type or grade of the material tested. While DSR testing has been quite an advance over previous methods, it still lacked the ability to determine quantitatively the fundamental viscoelastic properties of the material.
It would be desirable to develop an improved accuracy DSR device and to combine such a DSR device with the means to interpret the results to determine the fundamental viscoelastic properties of a material.
As used herein the term "viscoelastic" is meant to include synthetic or natural rubbers, plastics, thermoplastics, elastomers and any other material that exhibits viscoelastic properties.